In the classical railroad air brake system, as developed from the Westinghouse air brake, the brake air line which passes from the lead locomotive and then from vehicle to vehicle down the length of the train provides two basic functions.
First, it is used to charge compressed air tanks in the railroad cars. The air stored in these tanks provides the energy needed to apply the brake shoes when a brake application is required. When the train is running normally, and no brake application is needed, a high pressure, typically 90 pounds for freight trains and 110 pounds for passenger trains, exists in the brake air line. The tanks in the cars are charged to the same pressure as the air in the brake air line.
Second, when a brake application is required, air is vented from the brake air line through a valve in the lead locomotive. This causes the pressure in the brake air line to be reduced by a controlled amount. In the cars of the train, this reduction of pressure is used as a signal to apply the brakes. In this event, valving in the cars utilizes the compressed air in the tanks to supply air to brake cylinders which apply force to the brake shoes so that the brakes are applied.
Although this classical air brake system was an enormous improvement over the art prior to it, it nevertheless had some features where improvement was possible. For one thing, the time needed for a pressure decrement to propagate down the line of cars in a long freight train is quite long, about a minute for a mile-long train. Hence, when a brake application is required, it takes some time before all the brakes in the train are applied. This is the case for both normal and emergency brake applications.
There are also some operational difficulties due to the fact that the same compressed air line is used both for charging the air tanks in the cars and for signalling brake application. When a brake application is made, some of the air in the air tanks in the cars is depleted. Since the pressure in the brake air line has been reduced to signal the brake application, there is not sufficient air pressure in the brake air line to recharge the air in the tanks. The air in the tanks cannot be recharged to its initial pressure while the brake line air pressure is low for applying brakes.
An unfortunate practice which stems from this aspect of traditional airbrake systems is the practice of power braking. This is a case in which an engineer, upon starting to descend a grade, makes a brake application which is too large, so that the train does not maintain its preferred speed. In this case, the engineer may apply engine power to maintain speed. Hence, the brakes and locomotive are working against each other. Fuel is expended and brake shoes are worn.
One method of decreasing the time needed for the brake application signal to reach remote portions of the train is to provide a radio link so that when a signal originates in a lead locomotive to apply brakes, a radio signal is transmitted which is received at some distance down the line of cars. Where the signal is received, it causes local venting of the brake line, so that brakes are applied more rapidly. The WABCO EPIC.RTM. brake system, for example, may be operated with a radio link for this purpose (Registered trademark of Westinghouse Airbrake Company).
The radio approach may have difficulty due to terrain which intervenes between the locomotive and the remote receiver causing the remote unit to fail to dump brakepipe pressure. Hence, some systems use electrical trainlines which are electric cables connected between cars down the length of the train. These can carry signals to electro-pneumatic valves which vent brakepipe air at many points along the train and, hence, cause a rapid and uniform brake application.
Either of these approaches, the radio link or the trainline link can improve the response time of the system. However, the operational difficulty of not being able to reduce a brake application which is in force is not solved by shortening the time needed for brakepipe pressure to drop.
One approach which has been used to keep the tanks charged is to employ two brake air lines. One of them always contains high pressure air for keeping the tanks charged. The other brake air line is used to signal the need for brake application by a pressure decrement. This approach suffers from the added labor and time needed for assembling a train since two hoses need to be connected when two cars are connected. This approach alone solves the problem of keeping the tanks charged but suffers from the time delay caused by the slow speed of the pressure decrement signal. However, a radio link, or a trainline link may be used to dump brakepipe air at railway vehicles remote from the lead locomotive.